Modern oxidizing air cleaning methods apply more often non thermal plasmas for destroying and degrading harmful substances, like smells, allergens and germs, These plasmas generate high reactive radicals , which are able to convert a wide variety of in air carried harmful substances at environmental conditions.
For more than 100 years the plasma treatment of air is known to work according to the principle of the dielectric discharge, known as well as barrier discharge. Large volume, non-thermal plasmas can be generated with dielectric barrier discharge. The dielectric material, preferably gas, is present between electrodes subjected to a high alternating voltage. The dielectric material hinders the movement of the electrons and finally interrupts it. The electrons are not only hindered but also dammed by the dielectric material, whereby an opposing field builds up in opposition to the outer activating field. The opposing field increases until the outer field and the opposing field compensate each other and the flow of electrons comes to a stop. The properties of the dielectric barrier material as well its form determine, beside the arrangement of the inner electrode and the outer electrode, appearance of the discharge, which is characterized by the occurrence of single discharges, the so called is filaments. These filaments appear in a huge number on a short term. They are distributed usually across the whole area of the electrode generating the plasma.
A so called “Siemens-tube” is used as a plasma generator for the oxidative treatment of air. The “Siemens-tube” is made up of a tube like dielectric material which is preferably quartz glass or borosilicate. The inner wall of the tube like dielectric material is lined with an inner electrode. The inner electrode is made of a conductive material is positioned close to the inner glass surface and preferably without an air gap. An outer electrode is arranged on the lateral surface of the dielectric material which is formed by a close fitting mesh, like a steel mesh. In case a high alternating voltage, lie 3-6 kV (kilo Volts) is applied to the inner and outer electrode, a dielectric barriers discharge occurs. In doing so, ions and ozone (O3 and O1) are generated.
During the plasma treatment o a gas flow with the “Siemens-tube” the air flow is guided through the tube like dielectric material. For this purpose the plasma is ignited at the inner electrode. The plasma at the inner electrode forms exclusively in the outer layers of the air flow which come directly in contact with the inner electrode. The much greater portion of the air flow reacts only with ozone and the oxygen ions, which are generated during discharge.
The outer layers of the air flow, which immediately are in contact with the plasma, be more effectively freed from pollutants, in particular of odors and bacteria, because in the plasma the highest energy in the form of free radicals, electrons and ions is present. Furthermore the plasma produces an intense UV radiation in the wavelength range smaller than 300 nm, which can efficiently break molecular bindings of air pollutants.
A problem with the plasma treatment of a gas flow according to the prior art is, that the inner electrode quickly get contaminated by the pollutants contained in the gas stream and thus loses effectiveness. A polluted inner electrode replacement is possible only with great effort and disruption of the plasma treatment.
The German patent application DE 197 17 160 A1 discloses a device for plasma-chemical conversion of exhaust gases, which creates a barrier discharge in the flowing gas. The device includes a reactor room that is flowed through in longitudinal direction from an inlet side to a discharge side of the exhaust gas to be treated. A dielectric that shields the reactor room of the first electrode is attached to the first electrode. A second electrode is placed on the dielectric inside the reactor and formed as a perforated sheet material, where the second electrode against is switchable the first electrode. The two electrodes will be connected for this purpose to an alternating voltage source. When applying the alternating voltage it comes to a gas discharge in the gap area between the structures of the perforated second electrode and the dielectric material. The gas discharge is therefore primarily generated near the surface of the dielectric material. The exhaust gas flowing by is swirled by the perforated sheet like material and briefly enters the excitation region of the plasma. The second limitation of the reactor room is formed by a wall. No discharge is formed between the dielectric material and the wall, closing off the reactor room. As a result, more or less large parts of the gas flow, depending on the distance of the wall, come not directly in contact with the plasma and flow therefore untreated through the reactor room.
The German patent application DE 100 26 725 A1 describes a exhaust gas purification device which has a current passageway formed between a pair of discharge electrodes. A high voltage producing device produces a high voltage between the electrodes to purify exhaust gas emissions. An auxiliary electrode is arranged in the passageway with holes between the discharge electrode and the auxiliary electrode so that electrical discharges can be produced between the electrodes.
The US patent application U.S. 2008/056934 A1 shows that diffusive plasma is for effective treatment of contaminated air and material processing. Air is purified and disinfected by passing through the diffusive plasma device which includes a reactor or a plurality of reactors arranged in parallel or series and is energized by a high voltage alternating current power supply. The diffuser, being electrically isolated, provides extra nucleation sites to initiate discharges The addition of a diffuser, thereby, enhances the overall effectiveness of decomposing chemicals and destroying microbes to achieve high air treatment and material processing performance. The diffuser can be made of suitable filtering materials to additionally serve as a filter. By incorporating suitable catalytic materials with the diffuser, the reactor becomes a catalytic plasma reactor wherein the plasma environment provides enhanced catalytic functions.
The international patent application WO 2004/026461 A1 describes a non-thermal plasma reactor comprising two electrodes and a gas flow path through the reactor. At least one electrode is provided with a multiplicity of triple junctions on or within a surface of the electrode that faces the other electrode. One or both electrodes are covered by pots, stripes or portions of dielectric materials.
The international patent application WO 97/30274 A1 describes a device for the cleaning of exhaust gases from internal combustion engines, in particular a diesel exhaust soot filter. Said device has a discharge electrode, a counter electrode opposite thereto for electrical charging of the exhaust gas components, a ceramic structure with a circular cross-section and ducts extending there through in the direction of flow, and an internal electrode at high voltage.
This electrode is arranged on the inner cylinder wall of the ceramic structure and creates an electrical field at right angles to the ducts passing through said ceramic structure. The soot particles are deposited and oxidized on the walls of the ducts, and a separation is provided to prevent flow through the hollow internal space of the ceramic structure.
Japanese patent application JP 2002-346374 A shows a gas treater with a gas passage through which a gas containing a component to be treated flows. A discharge electrode is disposed in the passage and performing discharge for generating a low-temperature plasma. A functional member and a heater are provided, wherein the heater heats the member. The member is disposed in the passage for treating the component remaining in the gas treated with the discharge of the electrode and is made from a functional material containing a combustion oxidation catalyst.
European patent application EP 1 659 665 A1 describes a discharge device for generating a streamer discharge from the tip of a discharge electrode towards a counter electrode. The discharge electrode in the shape of a wire or rod is disposed substantially parallel to the counter electrode, whereby, even when the tip of the discharge electrode becomes worn out, the shape of the tip of the discharge electrode remains unchanged and the distance between the discharge electrode and the counter electrode remains unchanged.
The international patent application WO 97/09071 A1 provides a method for sterilization of objects. The object that should be sterilized is placed in a closed vessel. The closed is placed between two high voltage electrodes. Applying a high voltage between the said electrodes results in a conversion of oxygen in the air inside the closed vessel into ozone.
U.S. patent application 2003/180199 A1 relates to a plasma reactor for reducing noxious gas by using a plasma reaction, a production method for such a plasma reactor. An emission control apparatus is used for reducing noxious gas contained in exhaust gas of a vehicle by such a plasma reactor.